Add Ratio type

[GreasySlug]

Fixes #218.

Changes proposed in this PR:

• [x] Add Ratio type

  • [x] Add ratio.rs
    • [x] add
    • [x] sub
    • [x] div
    • [x] rem
    • [x] mul
    • [x] eq
    • [x] PartialEq
  • [x] Add _erg_ratio.py
    • [x] Ratio class
    • [x] RatioMut class
    • [x] Ratio.d.er

• TODO

  • [ ] integration tests
  • [ ] declare of Float type

Removed

These contents are excluded

• [ ] Add Complex type
  • [x] Implement complex type by using ratio.rs
  • [x] Add _erg_complex.py
  • [x] Add ComplexLit, Im token
  • [x] parse Im token
  • [x] convert Im token and num calc to ComplexLit
• [x] Improve Int, Nat, Bool type

Currently known bugs

• [x] import "consts" infinite loop? or so long runtime
• [x] tests/should_ok/structural.er
• [x] mut_array.er
• [ ] Cannot define Float type

• [ ] print! 0.3 runtime is 3/10 but test code eval is 0.3

  @mtshiba

[mtshiba]

The display of rational numbers is not consistent. It would be better to display them in the form numerator/denominator (e.g. 3/10).

>>> 3/10
Ratio(3, 10)
>>> 0.3
(3, 10)
>>> 2.3 % 2
Fraction(3, 10)